Rapid advancements in size and weight reductions of mobile information terminals such as mobile telephones, notebook computers, and PDAs in recent years have created a demand for higher capacity batteries as driving power sources for such devices. With their high energy density and high capacity, non-aqueous electrolyte secondary batteries, such as represented by lithium-ion secondary batteries, are widely utilized as the driving power sources for such mobile information terminals as mentioned above.
A non-aqueous electrolyte secondary battery as mentioned above generally employs a positive electrode comprising a lithium-containing transition metal composite oxide such as lithium cobalt oxide (LiCoO2), a negative electrode comprising metallic lithium, lithium alloy, or a carbon material such as graphite that is capable of intercalating and deintercalating lithium ions, and a non-aqueous electrolyte in which an electrolyte made of a lithium salt such as lithium fluoroborate (LiBF4) or lithium hexafluorophosphate (LiPF6) is dissolved in an organic solvent such as ethylene carbonate (EC) or diethyl carbonate (DEC). This kind of battery performs charge-discharge operations by transferring lithium ions between the positive and negative electrodes.
With the battery employing lithium cobalt oxide as its positive electrode material, the production cost tends to be high because of the use of cobalt, which is a rare resource the reserve of which is limited and is expensive. Moreover, the battery employing lithium cobalt oxide has an additional problem of low thermal stability because, when the battery in a charged condition is brought to a temperature that is unusually high, oxygen may be released from the positive electrode and the electrolyte may be burnt. For these reasons, use of lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2), or the like has been investigated as an alternative positive electrode material to lithium cobalt oxide. When lithium manganese oxide is used, however, a sufficient discharge capacity may not be obtained, and moreover, when the battery temperature becomes high, manganese dissolves in the electrolyte solution and deposits on the negative electrode, degrading cycle performance. On the other hand, the use of lithium nickel oxide has problems such as degradation in discharge voltage.
In view of these problems, olivine-type lithium phosphate such as lithium iron phosphate has in recent years drawn considerable attention as an alternative positive electrode material to lithium cobalt oxide. The olivine-type lithium phosphate is a lithium composite compound represented by the general formula LixM1−(d+t+q+r)DdTtQqRr(XO4) (wherein M includes at least one element among Fe, Mn, Co, Ti, and Ni; X includes at least one element among Si, S, P, and V; D is selected from bivalent ions and D=Mg2+, Ni2+, Co2+, Zn2+, or Cu2+; T is selected from trivalent ions and T=Al3+, Ti3+, Cr3+, Fe3+, Mn3+, Ga3+, Zn3+, or V3+; Q is selected from tetravalent ions and Q=Ti4+, Ge4+, Sn4+, or V4+; R is selected from pentavalent ions and R═V5+, Nb5+, or Ta5+; 0≦x≦1; and 0≦d, t, q, and r≦1), and it shows varied working voltages depending on the type of the metal element M, which serves as the core.
Accordingly, it has an advantage that battery voltage can be freely set by appropriately selecting the metal element M, which serves as the core. Another advantage is that since its theoretical capacity is relatively high, about 140 mAh/g to 170 mAh/g, battery capacity per unit mass can be made large. Furthermore, lithium iron phosphate (LiFePO4), which contains iron as M in the general formula, has an advantage that the production cost of batteries can be significantly lowered because of the use of iron, which is produced abundantly and is low in cost.
Nevertheless, in order to use olivine-type lithium phosphate as a positive electrode active material for the non-aqueous electrolyte battery, there are still problems to overcome. Particularly serious issues are as follows. Specifically, the olivine-type lithium phosphate is slow in the lithium intercalation and deintercalation reaction during battery charge-discharge operations and is much lower in electronic conductivity than lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide (LiMn2O4), and the like. For this reason, the battery employing an olivine-type lithium phosphate shows an increase in polarization particularly in high-rate discharging, considerably degrading battery performance.
In order to solve this problem, Patent Reference 1 proposes the use of a positive electrode active material in which the particle size of primary particle of LiFePO4 is very small 3.1 μm or less and the specific surface area is sufficiently large.
It is thought that the use of the positive electrode active material proposed in Patent Reference 1 increases the contact area with the conductive agent and improves the electronic conductivity of the positive electrode active material, but the use of a positive electrode active material with a small particle size has a problem of lowering the filling density of the positive electrode active material, thus lowering the energy density of the battery as a whole. Another problem is that the olivine-type lithium phosphate shows a lower degree of adherence with the metal foil that serves as the positive electrode current collector, so it tends to easily peel off from the positive electrode current collector even if a binder agent is mixed therein. To date, Patent Reference 2 below proposes the technique of preventing the peeling-off of the positive electrode active material form the positive electrode current collector by providing a positive electrode current collector having a large surface roughness with a positive electrode mixture.    [Patent Reference 1] Japanese Published Unexamined Patent Application No. 2002-110162    [Patent Reference 2] Japanese Published Unexamined Patent Application No. 5-6766